Electrical cables which are used in oil wells must be able to survive and perform satisfactorily under conditions of heat and mechanical duress which can be extreme. Ambient temperatures in wells are often high and the I.sup.2 R losses in the cable itself add to the existing heat. The service life of a cable is known to be inversely related to the temperature at which it operates. Thus, it is important to be able to remove heat from the cable while it is in its operating environment.
Cables are subjected to mechanical stresses in several ways. It is common practice to attach cables to oil pump pipes to be lowered into a well using bands which can, and do, crush the cables, seriously degrading the effectiveness of the cable insulation and strength. The cables are also subjected to axial tension and lateral impact during use.
It is therefore conventional to provide such cables with external metal armor and to enclose the individual conductors within layers of materials chosen to enhance the insulation and strength characteristics of the cable, but such measures are not adequate to provide the necessary protection.
An additional problem arises as a result of down-hole pressures, which can be in the hundreds or thousands of pounds per square inch, to which the cables are subjected. It appears that the insulation surrounding the conductors in a cable unavoidably has small pores into which gas is forced at these high pressures over a period of time. Then, when the cable is rather quickly extracted from the well, there is not sufficient time for decompression to occur, i.e., for the intrapore pressure to bleed off. As a result, the insulation tends to expand like a balloon and can rupture, rendering the cable useless thereafter.
Examples of prior art cables for various uses are found in the following U.S. Pat. Nos:
1,740,076--Delon
2,107,031--Evans
2,483,301--Roberds
2,810,010--Davey
3,102,740--Plummer
3,106,600--Crosby
3,409,731--Fink et al.
3,621,108--Cleaveland
3,681,509--Johnston et al.
3,798,346--Kreuzer
3,843,568--Woodland et al.
4,105,860--Ball
4,166,920--Friedrich et al.